The road from Port-au-Prince to the well-heeled suburb of Pétionville on the outskirts of the Haitian capital winds up a steep ridge, passing stalls selling painted metalwork, wooden furniture and tourist art, reminders of a more prosperous time. There is a ruined lottery office, where the agent still continues with his business. It is nothing but a solitary wall and a window now, through which the hills are visible beyond. Around halfway up the well-maintained carriageway you reach broken walls, enclosing what is left of the Montana hotel, where 60 people died. To the right, the ground falls quickly away into the deep green cleft that is the Vallée de Bourdon, a sprawling and impoverished community that clings to the river at the bottom.

The locations of the Montana and the Vallée de Bourdon mirror the social positions they once occupied in Haiti. Where the Montana sat at the apex – a haven for cocktail-sipping diplomats, aid workers and wealthy gangsters – the valley below was its polar opposite. Originally an informal shantytown for Port-au-Prince's poorest, over the years it had hardened into something more solid in the ravine's depths – a home for almost 6,000 people. Then the earthquake came and destroyed them both.

To reach the valley floor, you descend concrete steps that lead to the shingle riverbed, which is crossed by a small footbridge. The first time I walk down into the valley, a few weeks after the earthquake, I see a handwritten sign in English. One of many begging for help, this one states that the valley has been "forgotten". Even amid the devastation I have seen in the city, the valley's claustrophobically steep slopes seem to magnify the devastation. Despite its proximity to Port-au-Prince, this place has always been at risk, from landslides and flash floods when the rains come. So when the earthquake struck on 12 January, it hit the valley hard; residents tell me 250 people died in the ruins.

On the terraces visible on the far side of the river, roofs have fallen and walls caved in. But it is only at the bottom, when I am able to turn around and look back along the path, that the worst is revealed. One area in particular draws my attention: a huge section of hill that peeled off like an Antarctic spur calving icebergs, hurling the blocky houses built on it towards the river. As they fell, they shunted into and through each other, crushing everyone inside.

At the bottom of the stairs, I am approached by a muscular and handsome man, mustachioed, with a large silver crucifix on a chain around his neck. When he speaks, a gold tooth is visible. I notice he has a plastic-wrapped packet in one hand.

Wilson Octaveus, aged 47, fires words at me so fast that at first it is difficult to comprehend. But I understand when he opens the packet to show me photographs of eight members of his household – including three of his 12 children and his wife – killed with six others when his house fell down. He points at their faces, reciting their names. Cindy, 16, his wife Gertrude-Jean, Wistandelle, aged eight, and Milauva, aged three. He asks me to come to see the ruins of the home he built, leading the way through the crevassed maze of collapsed concrete blocks that hang above the filthy stream where pigs wade in the shallows and the women wash themselves.

His story emerges in fragments. The way others defer to him when we first meet, on an earth shoulder by the river on which he has built a wooden shelter, suggests that in this poor community he is a person of some note. He tells me he arrived in the valley in 1984 as part of a vast movement of people from the countryside to Haiti's cities, swelling Port-au-Prince from a city of several hundred thousand to 3 million, most living in slums like the Vallée de Bourdon. Places primed for destruction.

A homeowner in a country where the majority of people rent, Wilson earned extra money from letting out rooms in the properties he built. He mentions various jobs and sources of income: farming and working as a mason, even acting as an informal nurse and selling medicines to the valley. I struggle for a while to find a definition for his status. He is not among the 5% in Haiti who control the land, the money and the businesses. Instead, in the world of the very poor, he is among the marginally less impoverished. At least he was before 12 January.

He leads the way among the tottering remains of houses to reach the most devastated area – a steep fan of rubble, bristling with pink-painted flakes of walls, scattered with clothes and bits of furniture, broken sinks and toys, pages from schoolbooks. He halts and, as he begins to speak, I notice Wilson is standing over a human skull pulled out of the rubble, stripped of its flesh by dogs and other animals. As my eyes acclimatise to the jumble of shapes in the fallen masonry and junk at his feet, I realise there are not one but two skulls and a section of jawbone. He explains they belong to two of his children and his wife. He says it is all he could recover from burrowing in the ruins.

"This was a big house, one above came down and took another out and then hit mine. This is where I lived. This is where I was able to make a living. This is where I rented rooms out. Now I've got nothing left."

Wilson picks up a stick and hooks one of the skulls through the eye socket. It is an act that should be ugly but strikes me as tender in its own strange way. "Look. Do you see?" he asks hesitatingly. "That's a girl called… that's my daughter, Cindy. This" – he indicates another of the skulls – "is my wife's head. It has been eaten by the dogs." He finds the corresponding photographs from among his packet of demolished memories.

Wilson does not cry. Instead, he seems wired with grief, fidgeting as if on a powerful stimulant. The tears I'll see later when I meet him again by chance. He is staggering down from the main road with Shirley, 26, one of his surviving children, slung across one shoulder. Her ankle was crushed when the earthquake happened. Shirley, her lower leg a mess of metal pins, tells me she was in a friend's house when her mother and three siblings died in Wilson's place. She was one of more than 300,000 injured, crushed and smashed, crippled and broken. Victims of traumatic amputation.

Wilson leads us to his shack, bigger and better than most I have seen. He has found a cable, hooked it to the grid and wired in electricity for when the power is occasionally working. There is a proper bed and chairs to sit on. Over the months the shelters built from branches, scraps of recovered wood and corrugated iron will spread around Wilson's new home. They will breed in every open space, being transformed by their occupants from cloth shelters to wooden huts to iron-roofed structures. Eventually, here in the valley, a small settlement will grow, shelter for some of the 1.5 million Haitians made homeless.

Wilson Octaveus was four hours' drive away, outside the earthquake zone, the day catastrophe came to the valley. I sense a guilt that he feels about his absence. "The earthquake came and did what it did," he says bitterly. "I wasn't here in Port-au-Prince. I was in Les Cayes. I was working there. I had 12 children to feed and to send to school."

Wilson heard about the earthquake on his mobile phone. "I tried calling the valley and managed to get through to one person. She told me the Vallée de Bourdon was destroyed. That I'd lost my family. At 3am, I walked to where I could find transport. I paid and paid. And I paid again until I got here at seven in the evening." He tried calling to his family but got no answer from the rubble.

Wilson takes out his pictures again to go through his grieving ritual of connection with his lost family. This time he does weep. "That's Gertrude-Jean, my wife, the mother of my children. She's 45 years old. She lived with the children, brought them into the world, fed them and spent time with them. Now she's gone. I've lost her. She'd always look after them. It hurts so much every time I look. I'll never see her again. I have the photograph, but when I look up, I don't see her alive."

When I see Wilson again, it's April, outside the Chaîne de L'Espoir hospital in Pétionville, a few kilometres from the Vallée de Bourdon, where he has taken Shirley. Two months have passed since our last meeting and a brisk South American doctor, one of the hundreds of foreign medical staff who have rushed to fill the gaps left in Haiti's healthcare system, has just removed the rods and screws that have held Shirley's ankle together while it mended.

While she waits to have her wounds freshly dressed, we sit outside the clinic. Most of the patients are survivors of the quake. Shirley's treatment is almost complete, though she can't yet put any weight upon her leg. But Wilson is downbeat, anxious about whether he can care for his daughter in his leaking shack.

"They really took care of Shirley. But now she is coming home and the job of looking after her is mine." He complains that, three months after the earthquake, he has still received virtually no aid and so must go back to the countryside to work for weeks at a time, tending the fields. When he comes home to his shelter he has his other children to look after, as well as other homeless family members. Sometimes, he says, there are 10 people in his shack.

Haiti's earthquake did not only kill and destroy, it wrought a fundamental social transformation, reversing the country's decades-long trend of rural depopulation. In the aftermath of the tremor, 600,000 residents of Port-au-Prince fled to the countryside, an exodus later encouraged by the government to take pressure off the wrecked capital. Wilson Octaveus is luckier than most. He has family still living near Les Cayes and knows how to farm.

But the post-earthquake countryside is not the same as the one that Wilson left as a young man. That countryside was ruined before the earthquake for different reasons. One was a gradual depopulation, driven by the advent of the "transistor revolution" – as my landlady, Elsie, calls it – the arrival not just of cheap radio, but programming in the Creole language, which made city life attractive. Another was the collapse of rice farming. Haiti was self-sufficient in rice – until President Clinton's administration insisted that the country drop its import tariffs to benefit US farmers. The result for Haitians was grinding rural poverty – and these poor and marginal communities are barely able to cope now with the massive urban flight back to the country.

When I can't reach Wilson by phone to arrange a last meeting with him in June, one of his daughters, Louna, aged 21, agrees to accompany us out of Port-au-Prince and through the earthquake-ravaged countryside to a remote village near the town of Cavaillon. There Wilson meets me, clutching his one-eyed fighting cock. We drive up to the patch of land he began renting in April, where he is growing bananas, yams, peas and mangoes. Most are months, if not a year, from being ready. The ripening mangoes, however, he is already sending back for his family to eat.

"The hardest thing for me since the earthquake has been trying to take care of the children. I can't even send them to school. I feel sometimes that I've become a beggar borrowing money and trying to get help from where I can. I don't feel happy any more. I used to have money. I used to go to the disco and the cinema. I don't…" Wilson struggles for a moment to express what he means: "I don't feel myself. I think about the things I need for things to be better. So I can be Wilson once again."